Fuel Cell

Fuel Cell

Fuel Cell is an electrochemical device that converts the chemical energy of a fuel into electrical energy directly without involving a combustion cycle. The first fuel cell was developed in 1839 by Sir William Robert Grove in England. Many companies are working on fuel cell to develop techniques which can reduce the cost. In India the fuel cell laboratory of Bharat Heavy Electricals Limited(BHEL) is developing Phosphoric Acid Fuel Cells since 1987.

Electrolysis is the process of producing hydrogen and oxygen from water by passing electricity through water. In a fuel cell, this process is reversed where hydrogen and oxygen gases are combined in an electrochemical cell to generate electricity and water. It is known as a cell since it has some similarities with a primary cell. Like a conventional primary cell it also has two electrodes and an electrolyte between them and produces DC power. Fuel cell is actually a static power conversion device. A basic hydrogen-oxygen fuel cell with phosphoric acid as electrolyte is shown in figure below.

Fuel Cell Operation

A fuel cell consists of two electrodes and an electrolyte. Fuel Cell electrodes are made porous in order to provide a large number of pockets where the gas, the electrolyte and the electrode are in contact for chemical reaction. Unlike normal battery, in a fuel cell both the reactants (eg. hydrogen and oxygen) are not permanently contained in the electrochemical cell, but are fed into it when electric power is required. In fuel cells, platinum coated graphite plates are used as electrodes separated by an electrolyte. In hydrogen- oxygen fuel cell, hydrogen gas is supplied at the anode side where the hydrogen molecules are effectively reduced to hydrogen ions which move into the electrolyte.

H₂  →   2H⁺  + 2e^-

Electrons so liberated at the anode build up a negative potential and travel towards the cathode through an externally connected circuit. Oxygen gas is supplied at the cathode where it is reduced by hydrogen ions to produce water.

4H⁺ + O₂ + 4e^-    →    2H₂O

Electrochemical reactions coupled with movement of hydrogen ions through the electrolyte generate an electric potential which causes electric current to flow through the load.

2H₂ + O₂ →  2H₂O  + Electric energy generated+ Heat energy released

This reaction is exothermic, which results in heating up the cell. A stream of air is circulated on the cathode side of the cell which absorbs enough heat to maintain outlet air and steam at 180 degree Celsius which is optimum for best performance of the cells.

Fuel cells generally run on hydrogen but any hydrogen rich material can also serve as a fuel source. This includes fossil fuels like methanol, ethanol, natural gas, petroleum distillates, liquid propane and gasified coal. Fuels containing hydrogen require a fuel processor that extracts hydrogen gas. Fuel cells can also run on several other fuels such as gas from landfills and wastewater treatment plants. Three basic fuel processor or reformer designs for fuel cells used in vehicles are steam processing, partial oxidation and auto thermal processing.

Energy flow diagram in a fuel cell

Steam reformer combines the fuel with steam by vaporizing them together at high temperature. Hydrogen is then separated using membranes. It is an endothermic process which means that energy is consumed by burning fuel or excess hydrogen from the outlet of the fuel stack.

Partial oxygen reformers combine the fuel with oxygen to produce hydrogen and carbon monoxide which then reacts with steam to produce more hydrogen. Partial oxidation releases heat which is utilized elsewhere in the system.

Auto thermal reformers combine the fuel with steam and oxygen and thus the reaction remains in heat balance. In general, both methanol and gasoline can be used in any of the three discussed reformer designs. Differences in the chemical nature of the fuels however can favour one design over another.

Fuel cells are ideal for power generation particularly for onsite service in areas that are inaccessible for grid supply. Renewable energy of the sun and wind can be utilized to generate hydrogen, by using power from PV solar cells or wind turbines from electrolysis of water. Thus hydrogen becomes an energy carrier that transports power from generation site to another location for use in fuel cells. If it is difficult to arrange supply of hydrogen fuel cells can easily operate on methanol. To generate one kilowatt of power, the fuel cell uses about 12 litres of hydrogen gas obtained from 0.6 litre of methanol that costs about 45 rupees per litre. The overall efficiency of the cell from fuel to electricity is about 60%. The efficiency of fuel cell directly depends upon its use, as in a chlor-alkali cell, besides electricity generation the waste heat, as a byproduct is also utilized usefully in industry.

Methanol is a liquid hydrocarbon fuel. It can directly be introduced from anode side of the fuel cell without converting to hydrogen. Such an arrangement is used in mobile applications of fuel cells such as in buses and in remote military sites where noise and smoke discharge such as in diesel generators are prohibited. Methanol can directly be oxidized to operate as fuel. In another method methanol is converted to hydrogen and carbon oxides. Hydrogen rich fuel gases so produced are fed into the fuel cells to generate electrical energy.

Advantages of fuel cell

1. It is less pollutant.
2. No charging is required.
3. It is quiet in operation as it is a static device.
4. The conversion efficiency is more due to direct single stage energy conversion.
5. Fuel cells have cogeneration capabilities. Ie., in addition to electric power fuel cell plants can also supply hot water, space heat and steam.
6. Fuel cell plant can be installed near the point of use, thus transmission and distribution losses can be avoided.
7. No cooling water is needed as required for the condenser of a conventional steam plant. The heat generated can be easily removed and discharged to the atmosphere or used locally.
8. Fuel cell plants are compact and require less space.
9. Large degree of modularity is available. The number of fuel cells can be increased as per the requirement.
10. Wide choices of fuels are available for fuel cells. Fuel cell can be operated with natural gas, ethanol, methanol, LPG and biogas etc.
11. Fuel cell power sources attain a high efficiency up to 55% whereas conventional thermal plants operate at 30% efficiency.

Classification of fuel cells

1. Based on the type of electrolyte
   • Phosphoric Acid Fuel cell(PAFC)
   • Alkaline Fuel Cell(AFC)
   • Polymer Electrolytic Memeberane Fuel Cell (PEMFC) or Solid Polymer Fuel Cell (SPFC) or Proton Exchange Membrane Fuel Cell (PEMFC)
   • Molten Carbonate Fuel Cell (MCFC)
   • Solid Oxide Fuel Cell (SOFC)
2. Based on the types of the fuel and oxidant
   • Hydrogen - Oxygen fuel cell
   • Hydrogen rich gas – air fuel cell
   • Hydrazine – Oxygen/hydrogen peroxide fuel cell
   • Ammonia – air fuel cell
   • Synthesis gas – air fuel cell
   • Hydrocarbon(gas)- air fuel cell
   • Hydrocarbon (liquid)- air fuel cell
3. Based on operating temperature
   • Low temperature fuel cell(below 150 degree celsius)
   • Medium temperature fuel cell(between 150-250 degree celsius)
   • High temperature fuel cell(between 250 - 800 degree celsius)
   • Very high temperature fuel cell(between 800- 1100 degree celsius)
4. Based on application
   • Fuel cell for space applications
   • Fuel cell for vehicle propulsion
   • Fuel cell for submarines
   • Fuel cell for defense applications
   • Fuel cell for commercial applications
5. Based on chemical nature of electrolyte
   • Acidic electrolyte type
   • Alkaline electrolyte type
   • Neutral electrolyte type

Alkaline Fuel Cells

Alkaline fuel cell which is the oldest fuel cell uses KOH as electrolyte with porous electrodes of carbon having nickel as the electro catalyst. Hydrogen is used as fuel and oxygen as oxidant. Its operating temperature is about 80mdegree Celsius. At anode, hydrogen gas reacts with hydroxide ions present in the electrolyte solution to form water and electrons are released.

H₂ +2(OH)^-   →    2H₂O+ 2e

Electrons so produced build up a negative potential and move towards the cathode through an externally connected circuit. At cathode the elcstrons are picked up by the oxygen atoms available there, react with water present in the electrolyte to form hydroxide (OH)^- ions which combines with hydrogen ions to form water.

1/2O₂ + H₂O+ 2e^-    →     2(OH)^-

H⁺ + (OH)^-    →     H₂O

With hydrogen and oxygen continuously supplied the fuel will steadily be oxidized by the ions produced in the process to generate electric power causing current to flow in the external circuit.

Polymer Electrolyte Membrane Fuel Cells(PEMFC)

PEMFC consist of a solid electrolyte which is an ion exchange membrane. The fuel used is hydrogen with air as the oxidant. The membrane is impermeable to gases but allows the hydrogen ions to move across it which makes the current to flow in the circuit. The cell operates at low temperature (80-100 degree celsius).

Polymer electrolyte membrane fuel cells are the electrochemical enrgy converters with an efficiency level of more than 60%. No gaseous environmental pollutants are produced like in internal combustion engines. This fuel cell operates at low temperature, so its start up and shutdown times are short. This type of fuel is especially suitable for small stationary applications in the power range below 1 MW.

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PAFC consists of an anode of porous graphite substrate with platinum alloy as the catalyst. The cathode is similar to the anode but made with a noble metal catalyst. The elcrtolyte matrix contains concentrated phosphoric acid and is located between anode and cathode. Stacking of individual cells is accomplished with a bipolar plate. This plate provides the electrical contact between the anode of one cell and the cathode of the adjacent cell. PAFC is alow temperature fuel cell and therefore it requires high purity hydrogen. This cell has an advantage that it can be used without any danger of poisioning the electrolyte, but CO has to be converted to avoid platinum poisioning.

Molten Carbonate Fuel Cells(MCFC)

A molten carbonate fuel cell needs a molten mixture of alkali carbonates as an electrolyte. Its operating temperature is about 650 degree celsius which allows the use of catalyst like nickel in the electrodes. High temperature keeps the carbonate electrolyte in liquid phase. The electrolyte is retained between two porous nickel electrodes. Being a high temperature fuel cell, there is an internal reforming system which takes place almost simultaneously with the electro chemical reaction. With the operating conditions in this cell, CO is oxidized through the water gas shift reaction to CO₂ with the production of hydrogen. The oxidizing agents for hydrogen are carbonate ions which are formed at the cathode. Thus the oxidant gas must contain CO₂. The byproducts of this cell are steam and carbon dioxide at high temperature of 545 degree celsius and a source of cogeneration. Thus the MCFC in addition to electricity also provides industrial process heat. Waste heat can generate steam in boiler which can drive a generator to supply additional electric power thus improving the total efficiency of the system.

Future Potential of Fuel Cells

It is predicted that large scale application of fuel cells is expected to enter into the growing market of high technology electronic devices like cellular phones, laptop computers and video cameras. Casio computers have released a notebook PC powered by fuel cell using methanol reforming. MTI Micro fuel cells, USA, have developed an external fuel cell for use in cell phones.

With chloralkali plants functioning, sufficient surplus hydrogen is available. This can partly be used for generating power by PAFC power plants. The real revolution is expected to come from the vehicular application of fuel cells. It will help the developed countries to propel their compliance of Kyoto protocol. That is to cut down the green house emissions to levels that are 5% below the 1990 level.

With all round applications, the generation of electricity from fuel cells is expected to jump to 15000MW in 2011 from a mere 75MW in 2001. It will cover all the three portable, stationary and vehicular market segments.